The present invention relates to particle accelerators. More particularly, the present invention relates to cost effective particle accelerators for applications including industrial applications such as radiography, cargo inspection and food sterilization, and also medical applications such as radiation therapy and imaging.
Particle accelerators operate by generating charged particles having a particular energy depending on the application. One exemplary particle accelerator is the standing-wave (SW) electron linear accelerator (linac) used in medical and industrial applications. FIG. 1A shows a simplified functional representation of a linac 100 which receives electrons from an electron gun 112. The electrons are accelerated to produce a high-energy electron beam 114 along an axial bore hole 140. Beam 114 is focused and accelerated in a series of accelerating cavities 120a, 120b . . . 120n by forces exerted on the electrons by microwave fields which are fed from an external microwave source 116 such as a klystron or a magnetron. The microwave fields resonate inside accelerating cavities 120a, 120b . . . 120n and also resonate inside auxiliary cavities 113a, 113b . . . 113m. 
For example, in some medical applications, the high-energy beam 114 produced by the linac 100 may be applied directly to a cancer therapy volume on a patient, or beam 114 may be used to generate photons (x-rays) 130 by colliding with a suitable target 122 such as tungsten or gold. The resulting x-ray beam 130 may be used to image cancerous tumors and/or to destroy the cancerous cells within the tumors by its ionizing effect (see Section 1.2, pages 29-32 of “Biomedical Particle Accelerators” by W. H. Scharf, AIP Press, 1994, ISBN 1-56396-089-3).
FIG. 1B illustrates the phases of the microwave fields along the accelerating cavities 120a, 120b . . . 120n and the auxiliary cavities 113a, 113b . . . 113m. SW linacs capable of generating electron beams with an energy level of up to 25 MeV require approximately 50 resonant cavities for stable operation. In addition, in order to accelerate the electrons efficiently along an axial bore 140, the microwave fields in the accelerating cavities 120a, 120b . . . 120n should have a phase difference of 180 degrees from one accelerating cavity to the adjacent accelerating cavity, e.g., from cavity 120a to cavity 120b. 
FIG. 2 is a graph 200 showing the different resonant frequency modes for an exemplary approximate model of a linac constituting of 51 resonant cavities with attendant frequency mode separation or spacing 251, 255 between adjacent modes. Around the π-mode 245, the mode frequency separation 255 is relatively small, and hence not ideal for stable operation of this exemplary linac. For this reason, the mode at the center of the graph, mode 241, also known as the π/2-mode, is generally the preferred mode of operation. This is because the π/2-mode provides the maximum frequency spacing, as measured along the vertical frequency axis, between adjacent modes, i.e. at mode frequency separation 251. Operation at the π/2-mode is conventionally realized through the use of a bi-periodic arrangement to ensure that the phase difference of the microwave fields is 180 degrees between adjacent accelerating cavities for efficient electron beam acceleration (see pages 76-82 of “Medical Electron Accelerators” by T. J. Karzmark, et al., McGraw-Hill, Inc., 1993, ISBN 0-07-105410-3) (see also pages 113-117 of T. P. Wangler, “Principles of RF Linear Accelerators”, John Wiley & Sons, Inc., ISBN 0-471-16814-9).
Referring back to FIG. 1A, in addition to the set of accelerating cavities 120a, 120b . . . 120n, a conventional bi-periodic structure for linac 100 requires an additional corresponding set of auxiliary cavities 113a, 113b . . . 113m. Each auxiliary cavity couples the microwave power to an adjacent pair of accelerating cavities through a corresponding pair of coupling irises. The number of frequency modes in a bi-periodic linac is equal to the number of the combined resonant cavities, i.e., the total number of the accelerating cavities 120a, 120b . . . 120n and the auxiliary cavities 113a, 113b . . . 113m. For efficient operation of the linac 100, all the constituent resonant cavities should resonate at specific frequencies to ensure synchrony between the electrons being accelerated in the accelerating cavities and the electromagnetic field oscillating in all the resonant cavities.
One conventional SW bi-periodic linac configuration is the side-coupled SW linac 300, shown in FIG. 3A, wherein the auxiliary cavities 313a, 313b . . . 313m are placed on the sides of the accelerating cavities 320a, 320b, . . . 320n, away from the axis of beam 314. Auxiliary cavity 313a is coupled to the adjacent accelerating cavities 320a, 320b through a corresponding pair of coupling irises 317a, 317b. Similarly, auxiliary cavity 313b is coupled to the adjacent accelerating cavity 320b through a coupling iris 318b. FIG. 3B is a cross-sectional view of linac 300 illustrating accelerating cavity 320a, auxiliary cavities 313a, 313b and coupling irises 317a, 318b. 
Another conventional SW bi-periodic linac configuration is the on-axis SW linac 400 shown in FIG. 4A, wherein the auxiliary cavities 413a, 413b . . . 413m are placed along the axis of beam 414. The auxiliary cavity 413a is coupled to the adjacent accelerating cavity 420a through a pair of irises 416a, 418a. Auxiliary cavity 413a is also coupled to adjacent accelerating cavity 420b through a pair of irises 417a, 419a. FIG. 4B is a cross-sectional view of linac 400 illustrating accelerating cavity 420b, auxiliary cavity 413b and coupling irises 416b, 418b. 
In one conventional method of manufacturing linacs, e.g., for linacs 100, 300, 400, constituent sub-assembly components are stacked and brazed together to ensure vacuum tight joints. These joints are also required to provide continuity of the linac inner walls hosting the microwave current associated with the electromagnetic fields hosted in the cavities. The brazing process involves the use of alloy brazing foils that are inserted into the joints between adjacent cavities. A brazing furnace provides heat to melt the brazing foils that solidify later to form the vacuum tight joints. During brazing, some of the molten brazing alloy can make its way inside the cavities, resulting in a change in the volume of the cavity(s) which in turn can change the resonant frequency characteristics of the linac.
For this reason, it is a common practice to manually tune the individual cavities after the brazing step in order to bring the frequencies of individual cavities to their nominal frequencies. This is usually done by a skilled tuning technician who has to affix the linac on a fixture, perform a series of measurements, and modify the cavities as needed by deforming the physical structure of each cavity until the desired frequency is achieved. This process is a time consuming and substantially increases the manufacturing cost of the linacs.
Hence there is a need for improved linacs which are less costly to manufacture, more efficient to operate and more compact in size.